Oxygen free radicals or reactive oxygen species (ROS) are highly reactive species which are known to be the major factor in cell injury via oxidation and subsequent function impairment of lipids, proteins, and nucleic acids. Indeed, active oxygen has been suggested as a major cause of aging and several diseases including cancer. ROS in particular are known to damage mitochondria. Oxidative damage to mitochondria is considered to be a major factor in cellular aging and ultimate cell death.
A mitochondrion (singular of mitochondria) is part of every cell in the body that contains genetic material. Indeed, they are found in the cells of all eukaryotes. Mitochondria are responsible for processing oxygen and converting substances from foods into energy essential for cellular functions. Mitochondria produce energy in the form of adenosine triphosphate (ATP), which is then transported to the cytoplasm of a cell for use in numerous cell functions. Mitochondria are known as the powerhouses of the cell because the ATP they produce supplies approximately 90 percent of the metabolic energy used by multi-cellular creatures.
The role of mitochondria in oxygen metabolism makes them prime targets for damage from oxygen radicals. Specifically, the mitochondrial respiratory chain (i.e. the electron transport chain) has been recognized as a major intracellular source of ROS. Formation of ATP in the mitochondria results in release of highly reactive superoxide free radicals, which can transform into other ROS such as hydrogen peroxide and hydroxyl radical. Cells have mechanisms to cope with this oxidative stress, but the efficiency of coping declines with age and the influence of extrinsic factors such as stress. Eventually mitochondria become inefficient in their ability to produce ATP leading to a loss of cellular function and often even cell death.
ROS are capable of causing damage to mitochondria by structural degradation of proteins and lipids within the inner mitochondrial membrane. One of the most damaging ROS species is the hydroxyl radical which causes lipid peroxidation. As lipid peroxidation increases over time, one of the major lipids in the inner mitochondrial membrane, cardiolipin, undergoes structural changes. These structural changes result in damage to the inner membrane and associated cardiolipin-protein interactions, which are critical to electron transport. For example, cytochrome c attaches to cardiolipin in a healthy, normal functioning mitochondrion. However, when cardiolipin is degraded, cytochrome c is released, which in turn triggers the cascade of events leading to programmed cell death.
It has been suggested that exogenous addition of cardiolipin may improve overall mitochondrial dysfunction. However, numerous difficulties have been encountered when attempting to exogenously deliver cardiolipin. Cardiolipin is unstable and extremely susceptible to oxidative degradation. For example, human cardiolipin contains polyunsaturated fatty acids, with over 85% belonging to the linoleic acid series. Unlike saturated or monounsaturated fatty acids, polyunsaturated fatty acids such as linoleic acid are readily degraded by ROS. In addition, simple topical application of cardiolipin would not be expected to penetrate to the inner mitochondrial membrane in a complex fully functional cell based model, and particularly in vivo because of oxidative breakdown of cardiolipin and barriers to transport.
There currently are several other ways to address mitochondrial dysfunction but each is limited in that they do not address the most critical failure associated with mitochondrial aging. For example, one proposed solution has aimed at increasing ATP production by providing nothing more than a substrate for ATP production but this solution completely fails to address overall mitochondrial dysfunction. Another proposed solution for mitochondrial dysfunction is based on exogenous addition of antioxidants. This approach has achieved some level of success by preserving the current state and function of the mitochondria. However, this approach does not address repair of existing damage and therefore is not an ideal solution. Yet another example of a solution is NeoLipid®, specifically the Lipid Conjugate Gemcitabine (cardiolipin conjugate gemcitabine) available from NeoPharm (Waukegan, Ill.). NeoLipid® is a cationically modified cardiolipin embedded in a phosphatidylcholine liposome. However, even this solution is not ideal for a topical treatment aimed at treating mitochondrial dysfunction.